The main purpose of the study is to determine whether it is better to give subjects fluconazole to prevent thrush from coming back or to wait and treat each episode of thrush. Also the effect of each strategy on the development of thrush in other places such as esophagus and vagina; conditions that put HIV-infected persons at risk for yeast infections; what happens to those who do not respond to treatment; and the effect each strategy will have on the quality of subjects' lives.